I have a Peavey Ultra Series 120 watt head powering a Marshall jcm900 half stack. For some reason after the amp is on for a while it makes a loud humming noise. I changed the tubes in the head and that didnt solve the problem. Anyone have any suggestions?
Brandon
nitekast@nitekast.com
www.nitekast.com
Guitar Head Humming Problem
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Craven Sound
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lonewolf
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Oh, OK...that's a 1960 cabinet...best ever made. I was thinking JCM900 amplifier, but I guess they also put those little brass plates on the 1960 cabs that said JCM900 Lead 1960.nitekast wrote:I'm not sure what cycle it is. It's a peavey head into a marshall 4x12 cab
...Oh, the freedom of the day that yielded to no rule or time...
Some things to try...
- Is the amp always in the same spot when it hums? There may be another piece of electrical equipment close by inducing the hum. I see you've tried other outlets, but did you physically move the amp to another spot, or just use an extension cord?
- Is there a build up of dust in the amp? There are high enough voltages in a tube amp that when the humidity gets high, heavy dust build up can start to conduct. If it has a fat layer of dust in it, clean it out. UNPLUG IT FIRST.
Other things (ONLY RECOMMENDED FOR EXPERIENCED REPAIR TECHS... THE HIGH VOLTAGE IN TUBE AMPS CAN KILL YOU, AND IS STILL PRESENT IN AMPS THAT HAVEN'T BEEN PLUGGED IN FOR YEARS):
- Bad filter caps, as Lonewolf pointed out. Actually every electrolytic cap in a tube amp should be replaced every 10 years.
- The 6.3 VAC supply to the tube filaments may be unbalanced relative to ground. It depends on the circuitry that Peavy uses. Good quality amps use a center-tapped transformer which will almost always be balanced (unless it's severely damaged), but some other manufacturers use 2 resistors as a voltage divider. If one of the resistors is damaged or out of spec, it will cause hum. Damaged resistors can also get worse from heat, so the hum could increase with the temp inside the amp. Other manufacturers use a variable resistor across the 6.3 VAC, and use the center tap to divide the voltage. This makes the balance adjustable, but much more susceptible to heat, time, abuse and misadjustment.
- Damaged series dropping resistors on the filter caps. Another way that the filament voltage can become unbalanced.
- Is the amp always in the same spot when it hums? There may be another piece of electrical equipment close by inducing the hum. I see you've tried other outlets, but did you physically move the amp to another spot, or just use an extension cord?
- Is there a build up of dust in the amp? There are high enough voltages in a tube amp that when the humidity gets high, heavy dust build up can start to conduct. If it has a fat layer of dust in it, clean it out. UNPLUG IT FIRST.
Other things (ONLY RECOMMENDED FOR EXPERIENCED REPAIR TECHS... THE HIGH VOLTAGE IN TUBE AMPS CAN KILL YOU, AND IS STILL PRESENT IN AMPS THAT HAVEN'T BEEN PLUGGED IN FOR YEARS):
- Bad filter caps, as Lonewolf pointed out. Actually every electrolytic cap in a tube amp should be replaced every 10 years.
- The 6.3 VAC supply to the tube filaments may be unbalanced relative to ground. It depends on the circuitry that Peavy uses. Good quality amps use a center-tapped transformer which will almost always be balanced (unless it's severely damaged), but some other manufacturers use 2 resistors as a voltage divider. If one of the resistors is damaged or out of spec, it will cause hum. Damaged resistors can also get worse from heat, so the hum could increase with the temp inside the amp. Other manufacturers use a variable resistor across the 6.3 VAC, and use the center tap to divide the voltage. This makes the balance adjustable, but much more susceptible to heat, time, abuse and misadjustment.
- Damaged series dropping resistors on the filter caps. Another way that the filament voltage can become unbalanced.
... and then the wheel fell off.